The structures of two post-synaptic neurotoxins, erabutoxin b and a from snake venoms were determined in this laboratory. Sequences of nearly 50 snake venom toxins are known and their toxicities measured. It has been possible to establish, at least tentatively, the structural and probable functional significance of the neurotoxin invariant and conservatively substituted residues and to provide a probable model of the reactive site region. In order to investigate this model, it is proposed, when it becomes possible, (preparation of crystals or at least quasi-ordered arrays) to determine the structure of the receptor-neurotoxin complex. The following studies are now in progress or proposed: 1. Refinement of the erabutoxin structure to 1.5A or higher resolution. The purpose of this is to use the erabutoxin structure to determine structures of some other neurotoxins by using the Molecular Replacement Method rather than heavy atom isomorphous replacement. 2. Determination of the structures of a few of those toxins which show normal toxicity and which differ from erabutoxin by multiple changes with changes in invariant positions. 3. Determination of the structure(s) of one or more long toxins to establish the stereo-chemical effect of change when a fifth disulfide bridge is introduced. (Before 2. and 3. can be inititated conditions must be found for the crystallization of toxins described as difficult or impossible to crystallize.)